A group of men attacked Russian opposition leader Alexei Navalny and members of his anti-corruption foundation on Tuesday at an airport in southern Russia.

The group, some wearing Cossack uniforms, initially poured milk over the activists before pushing Navalny and several of his colleagues to the ground, punching and kicking some of them. Among Navalny's group were women and his young son.

Navalny and six others were injured in the attack, his spokeswoman Kira Yarmysh wrote on Twitter, and one of them was hospitalized. Two police officers nearby took no action to stop the attack, she said.

Navalny tweeted that around 30 people took part in the attack.

The 39-year-old opposition leader and about 30 of his foundation's staffers were returning from a team-building weekend in the countryside and had arrived at the airport in the Black Sea coast town of Anapa on Tuesday morning when they were attacked.

Navalny has accused top government officials, including President Vladimir Putin, of graft. In April, Russia's main state television channel (Rossiya-1) broadcast a documentary alleging Navalny is an agent of British and U.S. intelligence.

In recent months, there have been several incidents in which Russian human rights activists and journalists have been verbally abused or physically attacked while traveling outside Moscow.

Cossacks, a paramilitary group dating back to the czarist period who today brand themselves as conservative patriots, helped Russia annex Crimea from Ukraine in 2014.

In 2014, members of a Cossack militia attacked the Pussy Riot punk group with whips in the Black Sea resort of Sochi as they tried to perform a song mocking Putin during the Winter Olympic Games.